highly resistant to corrosion - which is important since the structure may be in place for a long time.

Picture 3.1 Cranes can be made taller because of HSLA steel.

HSLA steel cars, trucks and bridges

The increased strength of HSLA steels means that structures can be built that contain less steel and are therefore lighter than they otherwise would be. This is an important feature for cars and trucks because it leads to fuel economy and to less damage to road surfaces. It is also important in the design of bridges since it means that the centre spans can be longer and need fewer supporting beams.

HSLA steel TV masts

An interesting advantage of HSLA steels is seen in the construction of tall television transmitter masts. The extra strength of the steel means that the sections making up the mast can be thinner and more stable because they offer less resistance to the wind.

The durability of HSLA steels

The extra strength of HSLA steel makes it more durable and gives it a longer useful life span. It is therefore selected for use in street lighting poles, oil storage tanks and earth moving equipment.

The steel angel

One type of HSLA steel is called COR-TEN because it is particularly resistant to corrosion. It can be exposed to the atmosphere in its bare condition which eliminates the need and cost of painting and re-painting.

You will see a famous application of corrosion resistant steel if you drive up the A1 near Gateshead. High above the road is the Angel of the North, a 54 metre wide sculpture. The techniques used in its construction owe more to the skills used in shipbuilding than in traditional sculpture.

Picture 3.2 Street lamps and car bodies benefit from the strength and durability of HSLA steel.

Why HSLA steels are strong

The starting point in achieving the desirable properties of structural steels is to get the composition of the alloy right. The extra strength can be achieved through various combinations of alloying elements which means that there can be some choice in the other properties that the steel will have.

A typical HSLA steel will contain about 0.15% carbon, 1.65% manganese and low levels (under 0.035%) of phosphorous and sulphur and additions of other elements:

Element

Percentage added

Effect on properties

Copper

0.2  1.5

Improves atmospheric corrosion resistance

Nickel

At least half of the copper content

Benefits surface quality

Niobium

0.02

Increased tensile strength and yield point

Nitrogen

0.003  0.012

Contributes to strength and can improve weldability

Vanadium

Up to 0.12

Increases strength without reducing weldability

Chromium, molybdenum, silicon and zirconium may also be added.

Microstructure of HSLA steels

The strength of all steels, including HSLA steels, comes from their microstructure. Strength is increased by: